1. Field of the Invention
The present invention relates to a power converter, and more particularly, to a control circuit used for limiting the output power of a switching power converter.
2. Description of Related Art
The switching power control is a conventional technology used in a power converter to control and regulate the output power. Various protection functions, such as over-voltage and over-current protections, are built-in in the power converter to protect the power converter and circuits connected thereto from permanent damage. The function of output power limit is generally used for overload and short-circuit protection. Referring to FIG. 1, a traditional power converter circuit includes a feed-forward compensation. A control circuit 50 generates a control signal VG to regulate the output of the power converter in response to a feedback signal VFB. The control signal VG drives a switch 30 for switching a transformer 10. A resistor 35 that is connected in series with the switch 30 determines the maximum output power of the power converter. The resistor 35 turns the switching current of the transformer 10 to a current signal VS. The current signal is coupled to the control circuit 50. FIG. 2 shows the circuit schematic of the control circuit 50. If the current signal VS is greater than a maximum threshold Vlimit, the control circuit 50 disables the control signal VG, and it also restricts the maximum power output. The energy stored in an inductor is given by,
  ɛ  =                    1        2            ×      L      ×              I        2              =          P      ×      T      
where P is the maximum output power and can be expressed by,
                              I          P                =                                            V              IN                                      L              P                                ×                      t            ON                                              (        1        )                                P        =                                                            L                P                                            2                ×                T                                      ×                          I              P              2                                =                                                    V                IN                2                            ×                              t                ON                2                                                    2              ×                              L                P                            ×              T                                                          (        2        )            
In Equations (1) and (2), Ip and Lp are the primary current and the primary inductance respectively of the transformer 10, tON is the switch-on period of the control signal VG while the switch 30 is switched on, and T is the switching period of the control signal. From Equation (2), we find that the output power varies as the input voltage VIN varies. If the safety regulations are taken into consideration, the input voltage ranges between 90VAC and 264VAC, wherein the out power limit of the power converter in high line voltage is many times higher than the output power limit in low line voltage. Although the output voltage (power) keeps a constant by automatically adjusting tON through a feedback loop, the maximum tON is restricted when the current signal VS is greater than or equal to Vlimit (Ip×Rs≧Vlimit, where Rs is the resistance of the resistor 35). Furthermore, the maximum output power is also affected by the delay time tD of the control circuit. There is a delay time tD from the moment the voltage in current signal VS is higher than the upper limit voltage (Ip×Rs≧Vlimit) to the moment the control signal VG is actually turned off. In the period of the delay time tD, the switch 30 is still turned on, and it keeps on delivering power as shown in FIG. 3. Therefore, the actual turned-on time of the control signal is equal to tON+tD, and the actual output power becomes as follows:
                    P        =                                            V              IN              2                        ×                                          (                                                      t                    ON                                    +                                      t                    D                                                  )                            2                                            2            ×                          L              P                        ×            T                                              (        3        )            
Although the tD time is short, generally within the range of 200 nsec˜350 nsec, the higher the operating frequency is, the more impact is caused by tD. Because the switching period T is short tD, it becomes more important relatively. The input voltage VIN should be compensated properly, such that the input voltage does not affect the maximum output power. Referring to FIG. 1, a resistor 20 is added between the input voltage VIN and the current signal VS for the feed-forward compensation. The function of the resistor 20 can compensate the difference caused by the input voltage VIN and the delay time tD. By properly selecting the resistance of the resistor 20, an identical output power limit for the low line and high line voltage input can be resulted. The following analysis illustrates how the resistor 20 is determined for reaching an identical output power limit.
With the incorporation of the resistor 20, the current signal VS can be expressed by,
                              V          S                =                              (                                                            V                  IN                                                  L                  P                                            ×                              R                S                            ×                              t                ON                                      )                    +                      (                                          V                IN                            ×              FFc                        )                                              (        4        )            
where FFc=R25/(R25+R20), R20 and R25 are the resistance of resistor 20 and 25 respectively.
The control signal VG is turned off when VS≧VLIMIT, and tD and t=tON+tD are substituted into the equation (4), VLIMIT can be expressed by,
                              V          LIMIT                =                              [                                                            V                  IN                                                  L                  P                                            ×                              R                S                            ×                              (                                  t                  -                                      t                    D                                                  )                                      ]                    +                      (                                          V                IN                            ×              FFc                        )                                              (        5        )            
The expression of “VIN×FFc=(VIN/LP)×RS×tD” is set to achieve the identical output power limit. We finally get
                    FFc        =                                            R              S                                      L              P                                ×                      t            D                                              (        6        )            
However, the resistor 20 causes the significant power consumption, especially in high line voltage input. The power consumption can be expressed by,
                              P          R                =                              V            IN            2                                R            20                                              (        7        )            
Besides, the high voltage existing in the resistor 20 causes the drawback for the component selection and a PCB layout.